This invention relates to an alkaline battery and more particularly to a sealing structure thereof in which an insulator such as glass or ceramic is integrally joined by fusion thereof to both metal members, one being electrically connected to an anode of the battery and the other being electrically connected to a cathode of the battery.
Such a sealing structure has improved sealing effects with high resistance against thermal shock by using selected materials which have desired coefficients of thermal expansion. However, when the fusion seals of the glass or ceramic utilized in the field of electronic industry are applied as they are, without any special considerations, to the sealing structure of an alkaline battery, several problems will be encountered due to electrochemical reactions of the alkaline electrolyte with the glass or ceramic.
Now, reference is made to the problems caused by fusion seal using glass. The first of the problems is that the glass is partially dissolved and molten into the alkaline electrolyte due to the electrochemical reactions. That is, as the alkaline electrolyte normally contains sodium hydroxide or potassium hydroxide, the glass surface contacting the electrolyte is gradually converted to sodium silicate or potassium silicate. Although the sealing glass generally employed is borosilicate glass, which has considerably higher insolubility against the aqueous solution of alkaline than the ordinary silicate glass, the sealing glass is partially dissolved by and molten into the alkaline electrolyte over a long period of time because the concentration of the sodium or potassium hydroxide in the electrolyte of the alkaline battery is high and usually in the range of 25 to 45 percents.
To prevent such dissolution, it is effective to add sodium silicate, potassium silicate or zinc oxide to the electrolyte such that the additive comes close to the saturation point thereof. However, such addition of sodium silicate or the like cannot completely prevent the dissolution of the glass.
In addition to the dissolution of silicon oxide into the electrolyte, there is another problem such that the glass surface contacting the alkaline electrolyte is converted to a solid phase of sodium silicate or potassium silicate, which forms an extraneous film distinct from glass. While the thickness of the film gradually increases, cracks are developed inside the film and, sometimes, the inner cracks reach the inner layer of glass to deteriorate the sealing effect of the glass.
Still another problem has been experienced at the area of fusion sealing between the glass and metal. The fusion seal of the glass and the metal is established through a thin layer of glass, in the range of 10 to 15 microns in thickness from the interface between the glass and the metal, in which the oxide of the metal or iron has been dissolved and diffused. This thin layer of glass containing the oxide of the metal is subjected to a severe electrochemical corrosion by the alkaline electrolyte, so that the alkaline electrolyte may leak out of the battery through the interface between the corroded glass and the metal in a compartively short period of time. This electrochemical corrosion will be considerably accelerated when the metal adjoining the thin layer of glass is in contact with the anode of the battery. For example, in case of a silver oxide battery wherein zinc oxide, silver oxide and potassium hydroxide are used as the anode, cathode and electrolyte, respectively, it could be noted that a short-circuit is formed in the battery with the zinc being an anode, the thin glass layer adjoining the metal surface electrically connected to the zinc being a cathode and potassium hydroxide being an electrolyte. In the thin glass layer acting as the cathode, the iron oxide is reduced on discharge, thus causing the electrochemical corrosion to proceed. The concentration of iron oxide in the thin glass layer increases toward the metal surface, so that a higher corrosion rate prevails in the vicinity of the metal surface and the leakage of the electrolyte takes place in this area at rather early times.
When the glass is directly exposed to the outside of the battery, a further problem has been experienced other than a cracking of the glass taken place due to impact, e.g. upon dropping of the battery. The water vapor in the atmosphere condenses on the glass surface to dissolve and melt out the sodium, potassium or the like from the glass, thus sometimes developing an ionically conductive liquid film. In such a case, a leakage current flows through the liquid film, thereby causing a self-discharge of the battery while in stock.